4. The accompanying diagram illustrates a taxi driver’s individual supply curve (assume that each taxi ride is the same distance).

a. Suppose the city sets the price of taxi rides at $4 per ride, and at $4 the taxi driver is able to sell as many taxi rides as he desires. What is this taxi driver’s producer surplus?

(Recall that the area of a right triangle is 1⁄2 × the height of the triangle × the base of the triangle.)